tonight, we turn to the hit and run capital of the country, los angeles. And a detective determined to track down the rivers who hit your loved ones and race from the scene. Cecilia vega tonight inside the unit out to find justice for the families, desperate to track those drivers down. Reporter: In l.A.'S smoldering, gritty skid row sits an unassuming concrete building -- the lapd central division. And inside sits a man, waiting for the phone to ring. Central traffic padilla, can I help you? Reporter: His name is detective felix padilla, and he's fighting a war against a special breed of killer. Detective padilla works hit and run crimes like these. Sometimes the victims survive, even walk away. Others are not so lucky. The things that I've done, the things that I've seen, the average person doesn't see in their lifetime. For better or worse. Reporter: It seems to be a dangerous time for bikers and pedestrians. According to a report out two weeks ago, nationally, pedestrian deaths by car are up 8% between 2009 and 2011. A study by the university of michigan said in l.A., pedestrian fatalities are three times the national average. And nearly half of all car accidents are hit and runs, according to the lapd. A fatal hit and run is padilla's specialty, like the case of carlos perez, struck in 2010 by an infiniti while crossing the road on his way to work. His body was found about a hundred feet down the road. Reporter: He was dragged. He was dragged. And nobody should die that way. Reporter: Is it fair to say that l.A. Has a hit and run epidemic? It's a serious problem. If you want to call it an epidemic, maybe. We'll come up with the solution. Reporter: After a decade on the job, padilla doesn't even flinch when watching a video like this. Whoa. I mean, that guy is completely airbor airborne. He is completely airborne. Reporter: The victim survived. His friend takes off running. And that's the good part. If he gets a partial license plate, at least we have something. Reporter: But with video typically not clear enough, hit and runs are notoriously hard to solve, needing forensic work and witnesses to step forward -- which they often don't do, as in this fatal hit and run. He's underneath the car and is rolled over. Reporter: And clearly, the two people who witnessed the whole thing just turn around and walk away. Yeah, exactly. Reporter: What percentage of accidents that you investigate do you think you've got a witness out there that's just not coming forward? Most of them. Reporter: According to padilla, the explosion of hit and runs and lack of witness help might have something to do with the high number of undocumented immigrants living in los angeles. Many afraid of any police interaction. They feel that, since they're without a license that, you know, it could jeopardize their status. Reporter: So, these cops hit the streets to make their cases. For clues, suspects and anyone willing to come forward. But every once in a while, a gumshoe gets lucky. Things come together. Take padilla's most recent serious hit and run case. The impact was so hard that it literally knocked the boots off her feet. Reporter: Late one night this past june, yolanda nunez was walking in a marked crosswalk to get a bite to eat, when a red honda mowed her down. Yolanda, good to meet you. How are you hanging in there? Well, the leg right here is swollen and it has stitches on this side and this side. Reporter: Looks like you got a gash on your eye, also? Yeah, right here. According to witnesses, the car was going freeway speed and just never braked, good-bye. Reporter: When a car is ghost, every little bit of evidence left behind helps, so padilla can track them down. "Csi," hit and run style. His team of forensic workers can take pieces of a broken mirror like these and match them to a suspect's car. Tread marks can i.D. The tires. Chips of paint can give a make and model. Most prized? Dna. Like the blood left by a suspect in these pictures. He was eventually tracked down in mexico. But it gets down to that level of detail, the investigation, where a paint chip could give you the clue to lead you to another avenue. Some type of idea of what kind of car it was. Reporter: From the type of car, they need the car. This is where those rare witnesses are key, and yolanda's case had them. One of the witnesses got the first number, the first letter of the license plate. Reporter: A partial plate still leaves a lot to be found, but sometimes the suspect does the work for him. Well, the following morning, the driver called, wanting to make a place report. Reporter: Padilla says the suspect called her insurance company first, then police, and failed to mention a person caused the damage. According to her lawyer, that's only because she didn't realize it. She looked down. When she looked back up, she had felt something and proceeded home. She thought a rock had been thrown at her so she was scared to stop. Reporter: Police paid her a visit, matched the plate numbers and description to yolanda's case and the car was impounded. Is there any way that you see in how a person could be driving this car and not know that it was a human being on the windshield? I don't see how you could not at least pull over to see what happened. Reporter: Fortunately, what happened was still all across the front of the car. Is that blood I'm looking at? It's blood, yes. We were able to take some touch dna from the hood. There was some skin, some clothing fabric, imbedded in the glass. And the blood that you see. Reporter: The evidence was brought to this lab and matched to yolanda. Bingo. The puzzle complete and the driver charged with felony hit and run. Potentially a couple of years in prison. She pleaded not guilty. It is very clear that she did not know she hit a person, and left the scene. Reporter: As for yolanda? What do you want to say to the person who hit you when you were crossing that street? What do I want to say? I don't know. Next time, be careful. Reporter: But a hollywood ending is hard to come by. Almost half of padilla's fatalities are unsolved. There are binders full of stories without a final chapter at his office. What are the hardest types of cases for you to investigate? The ones where you have no leads. All you have is a torn family, or a person who's had his beautiful car destroyed, or, you know, somebody's in the hospital. Reporter: Like the case of carless per -- carlos perez. All that was left were parts of that infiniti, a dead father and plenty of questions. Is the problem of hit and runs in l.A. So big that you will retire and the problem will be as it is today? I don't think so. I think if we let the public know that these crimes are so frequent that we need their assistance, the more they help us, the more cases we'll solve.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Woman walks out of jail over 2 years after fiance's kayak death: Part 1

Now Playing: Detectives believe they found motive for woman to kill her fiance: Part 2

Now Playing: Woman is charged in her fiance's kayak death: Part 3

Now Playing: Experts weigh in on prosecutors' murder theory in man's kayak death: Part 4

Now Playing: How woman convicted in fiance's kayak death looks to rebuild her life: Part 6

Now Playing: Woman pleads guilty to lesser charge in her fiance's kayak death: Part 5

Now Playing: The drama behind the scenes on 'Roseanne': Part 2

Now Playing: How 'Roseanne' broke many of the sitcom rules: Part 3

Now Playing: Saying goodbye to 'Roseanne' after a shocking final episode: Part 4

Now Playing: 'Roseanne' star Roseanne Barr says 'we're lucky' to have Trump as President

Now Playing: Coming together on the iconic 'Roseanne' couch: Part 6

Now Playing: How the 'Roseanne' cast signed up for the reboot: Part 5

Now Playing: How the original 'Roseanne' pilot came together: Part 1

Now Playing: The behind-the-scenes drama of 'Roseanne's' first season

Now Playing: How the original 'Roseanne' sitcom was started

Now Playing: 'Roseanne: The Return' - A Special 20/20 Event - Tonight at 10/9c on ABC

Now Playing: 'Roseanne' star on the first time she met co-star Laurie Metcalf

Now Playing: Lionel Richie on working with Diana Ross and how he wrote the song 'Endless Love'

Now Playing: Songwriter Diane Warren on how she came up with the idea for 'I Don't Wanna Miss a Thing'

Now Playing: 'Roseanne' star on how the show's set really 'felt like home'